Two fisherman from the Western Isles have been jailed for at least 18 years for murdering their teenage friend Liam Aitchison after a brawl nearly two years ago.

Johnathon MacKinnon and Stefan Millar, both 22, from Stornoway on the Isle of Lewis, were found guilty last month of beating and repeatedly stabbing Aitchison and dumping his partially clothed body in a disused building in November 2011.

The case against both men hinged on witness testimony about arguments with Aitchison and allegations by another prisoner that Millar had boasted of his part in the murder while he was on remand at Inverness prison. Millar took to the witness stand in his trial to deny being involved.

Although Aitchison was violently attacked, no forensic evidence, witnesses or murder weapons were found despite exhaustive searches of the house in the suburb of Plasterfield, Stornoway, where he was thought to have been killed.

Liam Aitchison. Photograph: Northern Constabulary/PA

As he sentenced the two men to life in prison at the high court in Edinburgh, with a minimum term of 18 years, Lord Kinclaven said Aitchison's death had caused his family "grief, stress and anguish".

The teenager's parents, Norrie and Claire Aitchison, had arrived at the court carrying their son's photograph and wearing jackets with the words "Shine on our Crazy Diamond. RIP Liam."

After sentencing, Claire Aitchison said: "We are happy that justice has been done and that these two brutal murderers will spend a long time in prison.

"We believe, however, that they are getting off lightly – life imprisonment should mean life.

"They killed an innocent boy in cold blood but, by the time they leave jail, they will still be young enough to make plans for the rest of their lives."

The sentences were welcomed by Police Scotland. Detective Inspector Andy Logan, one of the senior officers involved in the investigation, said: "The murder of Liam Aitchison was the first reported murder in the Western Isles for many years and caused considerable distress in such a small community.

"Liam's family have also had to cope with the loss of Liam in the most tragic of circumstances and hopefully today's sentences will help to reassure the community and help the family in coming to terms with their terrible loss."

In a statement following the hearing, Liam Murphy, the procurator fiscal for the north of Scotland, said: "Thankfully, murders in this part of the country are very rare.

"I hope that these murder convictions reassure those communities that when we do have such crimes, justice will be done."